Electromagnetic-wave oscillator

ABSTRACT

An electromagnetic-wave oscillator includes a substrate, an EMW oscillating unit including a gain portion, an EMW resonance portion, an EMW radiating portion, and a ground (GND) portion, and a supplying unit for supplying electric power to the EMW oscillating unit. The ground portion regulates a predetermined reference electric potential for the gain portion, the EMW resonance portion, and the EMW radiating portion. The EMW oscillating unit is disposed on a first surface of the substrate. The supplying unit is disposed on a second surface of the substrate extending on an opposite side to the first surface. The EMW oscillating unit and the supplying unit are electrically connected via a penetrating electrode formed in the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electromagnetic-wave (EMW)oscillator. Particularly, the present invention relates to an EMWoscillator for emitting an electromagnetic wave at a frequency orfrequencies including at least a portion of a frequency range from 30GHz to 30 THz. The electromagnetic wave including at least a componentin the above frequency range is called a terahertz (THz) wave in thisspecification.

2. Description of the Related Background Art

In recent years, techniques using a THz wave in communications field,security field, medical field, and so forth have been energeticallyresearched and developed. The THz wave has characteristics ofpenetrating and straightly propagating through a substance. It is,therefore, possible to obtain inner information of a substance at highresolution by using a THz wave reflected by, or transmitted through thesubstance. Accordingly, various non-destructive or non-invasiveinspection techniques have been researched and developed.

Some examples of these inspection techniques include a technique forsecurely performing a seeing-through or perspective imaging of asubstance using a THz wave in place of X-rays, a spectroscopic techniquefor inspecting the bonding condition of a molecule by obtaining theabsorption spectrum and the complex dielectric constant of a substance,a technique for estimating the carrier density and the mobility of asuperconductive material, a technique for analyzing a bio-molecule, suchas DNA and proteins, etc.

The development of a THz-wave source is indispensable to put the abovetechniques into a practical use. To date, there have been developedTHz-wave generating techniques using laser apparatuses, such as aphotoconductive device capable of being excited by a femtosecond (fsec)laser light, and a THz-wave parametric oscillator using a non-linearoptical crystal. Further, there have also been developed THz-wavegenerating techniques using a small-sized electron vacuum tube, such asa backward-wave oscillator (BWO) and a gyrotron, and a large-sizedelectron beam accelerator, such as a free electron laser. According tothose techniques, the oscillating frequency is changeable, and theoutput power is large, so that those techniques are highly effective inparticular uses, such as the identification of fingerprint spectra ofvarious substances. However, in those techniques, the size of anapparatus increases, and hence its general or industrial use isrestricted.

The following oscillators have been developed as a small-sized radiationsource. For example, some oscillators are constructed by combiningactive elements using the negative resistance generated by the movementor transition of electrons in a semiconductor due to injection of acurrent thereinto, such as a Gunn diode and a resonance tunnel diode(RTD), with a variety of antennas (or resonance structures) In this way,a small-sized oscillator with a single oscillation frequency can berealized, though its output is low (especially low in an oscillator foremitting a THz wave). Therefore, such oscillators are expected to beapplied to various uses. Conventional examples of such a small-sizedoscillator will be described in the following.

“APL, Vol. 58 (20), p. 2291, 1991” discloses a small-sized oscillatorconstructed by the combination of an active element of a double barrierRTD with AlSb barrier layers (1.5 nm in thickness) and InAs quantum welllayers (6.4 nm in thickness) grown by a molecular beam epitaxy (MBE),and a square (300 microns*150 microns) waveguide serving as a resonancestructure. According to this referenced paper, oscillation at afrequency of 712 GHz is achieved by a single device, and its output is0.3 microwatt.

Further, “IEEE, Transactions on microwave theory and techniques, Vol.42, No. 4, 1994” discloses a planar integrated Gunn diode arrayconstructed by the combination of an active element of a GaAs Gunndiode, and a microstrip line (MSL) patch antenna serving as a resonancestructure.

FIG. 7 illustrates this planar integrated Gunn diode array. According tothis referenced paper, it is possible to collectively fabricate theactive element (negative resistance element), the resonance structure(antenna), and a DC (direct current) supplying portion (circuit andelectrode for performing the DC supply to the active element) on asubstrate, using a conventional semiconductor process. Accordingly,decrease in a size of the oscillator, increase in an output by arrayingthe structures, and improvement in oscillation characteristics areexpected. According to this referenced paper, oscillation at 12.423 GHzis achieved in a Gunn diode of a 4*1 linear array type (illustrated inFIG. 7), and oscillation at 12.395 GHz is achieved in a Gunn diode of a2*2 loop array type.

Furthermore, “Preliminary Papers at the 53rd JSAP (Japan Society ofApplied Physics)-related lecture, 2006 spring, No. 3, 23p-M-2” suggestsas follows. As a method for overcoming the problem of a low output powerin a range of the THz wave, it is expected that the output power of anoscillator in a range of 330 GHz can be increased by mutuallysynchronizing current injections into densely arrayed oscillatingelements.

In conventional THz-wave oscillators of the planar integrated type, itis necessary to arrange a DC supplying portion including elements, suchas a circuit, an extraction electrode, and a bonding wire, on asubstrate, in addition to an EMW radiating portion including elements,such as a negative resistance element, an antenna, an EMW resonanceportion. The size of the DC supplying portion is in the order ofapproximately 1 mm². It is therefore likely that the DC supplyingportion occupies a considerable space, and the flexibility of design ofthe EMW radiating portion on the substrate is reduced. Further, the DCsupplying portion and the EMW radiating portion are disposed on the samesubstrate, so a THz wave emitted from the radiating portion is likely tointerfere with the DC supplying portion. Also in this respect, theflexibility of design tends to be restricted. As a result, it is noteasy to densely array THz-wave oscillators of the planar integratedtype.

SUMMARY OF THE INVENTION

The present invention provides an electromagnetic-wave oscillatorcapable of addressing the above problems.

According to one aspect of the present invention, there is provided anelectromagnetic-wave oscillator, which includes a substrate, an EMWoscillating unit including a gain portion, an EMW resonance portion, anEMW radiating portion, and a ground (GND) portion, and a supplying unitfor supplying electric power to the EMW oscillating unit. The groundportion regulates a predetermined reference electric potential for thegain portion, the EMW resonance portion, and the EMW radiating portion.The EMW oscillating unit is disposed on a first surface of thesubstrate. The supplying unit is disposed on a second surface of thesubstrate extending on an opposite side to the first surface. The EMWoscillating unit and the supplying unit are electrically connected via apenetrating electrode formed in the substrate.

According to another aspect of the present invention, the EMWoscillating unit and the supplying unit are disposed on mutuallyopposite surfaces of the substrate, respectively, and the penetratingelectrode is formed in the substrate to connect the EMW oscillating unitand the supplying unit. Accordingly, the flexibility of design of theEMW oscillating unit on the substrate is increased, and the possibilityof interference between an electromagnetic wave radiated from the EMWradiating portion and the supplying unit is reduced.

Further, it becomes easy to dispose a curved lens plate, such as asemi-spherical lens plate, on the surface of the substrate. In thisstructure, it is possible to couple an electromagnetic wave radiatedfrom the oscillator to the lens plate, and appropriately shape a beam ofthe electromagnetic wave. Thus, the electromagnetic wave can beefficiently transmitted from the oscillator.

The features of the present invention will be more readily understood inconnection with the following detailed description of embodiments andexamples of the invention in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating a first embodiment of an EMWoscillator according to the present invention.

FIG. 2 is a cross-sectional view taken along line A-A′ of FIG. 1.

FIG. 3A is a plan view illustrating an upper plane of an array ofoscillators of the first embodiment.

FIG. 3B is a plan view illustrating a lower plane of the array ofoscillators of the first embodiment.

FIG. 4 is a cross-sectional view illustrating a second embodiment of anEMW oscillator according to the present invention.

FIG. 5 is a cross-sectional view illustrating a third embodiment of anEMW oscillator with a layered lens plate according to the presentinvention.

FIG. 6A is a plan view illustrating the third embodiment of anoscillator of a first type including arrayed EMW oscillating elementswith layered lens plates.

FIG. 6B is a plan view illustrating the third embodiment of anoscillator of a second type including arrayed EMW oscillating elementswith a layered lens plate.

FIG. 7 is a plan view illustrating a conventional oscillator includingarrayed EMW oscillating elements.

DESCRIPTION OF EMBODIMENTS

Embodiments of an EMW oscillator of the present invention willhereinafter be described with reference to the drawings.

A first embodiment of a THz-wave oscillator of the planar integratedtype will be described. FIG. 1 is a plan view of the first embodiment.FIG. 2 is a cross-sectional view taken along line A-A′ of FIG. 1. Aninstallation substrate 122 and electric wires 118 and 121, which aredepicted in FIG. 2, are omitted in FIG. 1. FIGS. 3A and 3B are upper andlower plan views illustrating an array of oscillators illustrated inFIGS. 1 and 2.

As illustrated in FIGS. 1 and 2, an oscillator 100 of this embodimentincludes an EMW oscillating unit 194 disposed on an upper surface of asubstrate 112, and a DC supplying unit 198, which is disposed on a lowersurface of the substrate 112, for supplying DC electric power to the EMWoscillating unit 194. The EMW oscillating unit 194 and the supplyingunit 198 are electrically connected via a penetrating electrode 102formed in the substrate 112. The EMW oscillating unit 194 includes aresonance tunnel diode (RTD) 101, a penetrating electrode 102 a, aresonator 103 of a microstrip line type (MSL resonator 103), an antenna104, a dielectric portion 107, and a GND layer 106. The MSL resonator103, the antenna 104, and the GND layer 106 constitute a resonancestructure.

The GND layer 106 is a ground portion for regulating a predeterminedreference electric potential for the RTD 101 that is a gain portion, theMSL resonator 103 that is an EMW resonance portion, and the antenna 104that is an EMW radiating portion. The MSL resonator 103 includes a phasestub 105, a first microstrip line (MSL) 109 a, and a second MSL 109 b.The substrate 112 includes penetrating electrode portions 102 b, 102 c,102 d and 102 e formed therein, a GND penetrating electrode 110, and ashield 113. The shield 113 is provided extending in a planar directionof the substrate 112, and a potential of the shield 113 is set to apredetermined ground electric potential. The DC supplying portion 198includes a lower DC signal line 190 and a lower DC electrode pad 117.

DC solder ball 114, DC electric wires 119 a, 119 b and 119 c, DC bondingwire 118, GND solder ball 120, GND electric wire 115, and GND bondingwire 121 are disposed on an installation substrate 122 provided underthe substrate 112.

More specifically, elements for oscillating an electromagnetic wave aredisposed on the upper surface of the substrate 112 in the oscillator100, while elements for supplying DC signal and ground signal requiredfor the EMW oscillation are disposed on the lower surface of thesubstrate 112. The penetrating electrode 102 for connecting thoseelements on the upper and lower surfaces of the substrate 112 isprovided in the substrate 112. The penetrating electrode 102 includesplural portions 102 a through 102 e. Members for electrically connectingthe oscillator 100 to external apparatuses including a DC electric powersource are arranged on the installation substrate 122.

Features and operation of the oscillator 100 of the first embodimentwill be described. When the DC signal is supplied to the RTD 101,induced emission of an electromagnetic wave occurs in the RTD 101. Thewavelength of the electromagnetic wave is an approximately singleoscillation wavelength λ (λ: the wavelength in vacuum) is a range of theTHz wave, that is acquired due to the negative resistance of the RTD101. The emitted electromagnetic wave is an electromagnetic wave with aneffective wavelength λ_(g) (λ_(g): the wavelength in the dielectricportion 107) in the dielectric portion 107, and propagates in an MSLpatch antenna 199. The MSL patch antenna 199 includes the MSL resonator103, the antenna 104, the GND portion 111, the dielectric portion 107,the RTD 101, and the like. The propagating electromagnetic waveresonates as a standing wave, and is amplified in the MSL patch antenna199.

Under some conditions, an electromagnetic wave with the oscillationwavelength is oscillated from the antenna 104. The oscillatingconditions of the electromagnetic wave are the following qualitativeconditions (1) and (2) that should be satisfied at the same time.

(1) The electromagnetic wave with the wavelength λ_(g) amplified in theoscillator 100 should exceed a certain threshold. In other words, an EMWgain obtained by the resonance should exceed EMW losses (including anelectric power consumed in the resonator) due to the structure of theoscillator 100.

(2) The phase matching of the resonating electromagnetic wave with thewavelength λ_(g) should be achieved. Namely, the phase difference shouldbe approximately zero.

In the structure of the first embodiment, the DC supplying portion 198for supplying electric power to the RTD 101 and elements for supplying asignal thereto from outside are arranged on the location opposite to thesurface of the substrate 112 on which the RTD 101 is arranged. Further,the penetrating electrode 102 is disposed at a place of the node of thestanding wave in the EMW radiating portion 194, i.e., at a locationwhere the amplitude of the standing electromagnetic field isapproximately zero. Thereby, members necessary for the EMW radiatingportion 194, such as the MSL resonator 103, are connected to the DCsupplying members, such as the lower DC signal line 190, arranged on thelower surface of the substrate 112 in a DC manner through thepenetrating electrode 102 formed in the substrate 112.

As a result, the DC supply to the RTD 101 can be performed with reducedinfluence on resonance characteristics of the standing wave, and the DCsignal line and bonding electrode necessary for the DC power supply canbe disposed on the lower surface of the substrate 112. Further, sincethe tolerance of layout on the upper surface of the substrate 112increases, the flexibility of design of the oscillator 100 largelyincreases. Thus, density and integration of the oscillator 100 can beenhanced. It is hence possible to provide a small-sized high-powerTHz-wave source. It is also preferable to dispose the GND penetratingelectrode 110 at a location of the node of the standing wave with thewavelength λ_(g). However, even when those penetrating electrodes 102and 110 are disposed at locations slightly shifting from the above node,somewhat advantageous effect can be obtained due to the arrangement inwhich the EMW radiating portion and the power supplying portion aredisposed on different surfaces of the substrate.

Furthermore, when the shield 113 for intercepting low-frequencyelectromagnetic wave is disposed in the substrate 112, or on its lowersurface, noises, such as low-frequency noise from the power supplyingside (i.e., the power supplying portion and the DC power source) can beintercepted or reduced. Accordingly, parasitic oscillation can bereduced, and the oscillation characteristics can be improved.

A more specific exemplified design of the oscillator 100 of the firstembodiment will be described referring to FIGS. 1, 2, 3A and 3B.Specific sizes of various portions are indicated in those figures (notto scale).

The oscillator 100 of the first embodiment can be designed such that aTHz wave at an oscillation frequency of 340 GHz, namely, λ=840 micronsand the effective wavelength λ_(g)=570 microns, can be oscillated. Thedielectric portion 107 can be formed of benzocyclobutene (BCB) with adielectric constant ε_(r) of 2.8.

The specific structure of the radiating portion 194 will be described.The EMW radiating portion 194 can be an oscillating element of a type inwhich the RTD 101 is used as an active element, and the MSL patchantenna 199 is used as a passive element. The EMW radiating portion 194can comprise RTD 101, penetrating electrode 102 a, MSL resonator 103,antenna 104, dielectric portion 107, and GND layer 106. The MSLresonator 103 can include phase stub 105, MSL 109 a, and MSL 109 b.

In the EMW radiating portion 194, the RTD 101 is used as the negativeresistance element. The RTD 101 can include an active layer with atriple barrier quantum well structure including a hetero junction ofInGaAs/InAlAs, and upper and lower contact layers of n⁺-InGaAs in whichSi is highly doped. The structure of the active layer is as follows fromthe upper side of the oscillator 100: InGaAs (5.0 nm in thickness);InAlAs (2.66 nm); InGaAs (5.61 nm); InAlAs (2.66 nm); InGaAs (7.67 nm);InAlAs (2.66 nm); and InGaAs (5.0 nm). With respect of estimation of itscurrent-voltage characteristic, −4.1 mS of the differential negativeconductance (i.e., an inverse of the differential negative resistance)is confirmed.

In the MSL patch antenna 199 serving as the passive element, Au/Ti (3kÅ/0.3 kÅ) can be used as the MSL resonator 103 and the antenna 104, aBCB with a thickness of 3 microns can be used as the dielectric portion107, and Au/Cr (3 kÅ/0.5 kÅ) can be used as the GND layer 106. Withrespect to the MSL patch antenna 199, the negative conductance and theoscillation wavelength can have the above values, respectively, and thecharacteristic impedance can be 50Ω. According to a calculation usingthe high-frequency electromagnetic field simulator, the aboveconstruction with sizes indicated in FIGS. 1, 2, 3A and 3B (not toscale) is designed such that conditions for oscillation of a THz wave at340 GHz are satisfied.

Structures of the DC supplying portion 198 for supplying the DC signal,and the substrate 112 including the penetrating electrode 102 will nowbe described.

The substrate 112 can be a Si substrate whose fine processing can bereadily performed using conventional semiconductor processes. In thesubstrate 112, there are disposed the penetrating electrodes 102 bthrough 102 e for the DC power supply, the GND penetrating electrodes110 a through 110 c for the GND supply, and the shield 113 set at theGND electric potential. The shield 113 can be provided in the form of afilm extending in the planar direction of the substrate 112 and set tothe GND electric potential.

The penetrating electrode 102 a is connected to the penetratingelectrodes 102 c, 102 d and 102 e via the penetrating electrode 102 b.Those electrodes 102 a through 102 e are further connected to the lowerDC signal line 190 disposed on the lower surface of the substrate 112,and connected to the DC power source (not shown) via the DC solder ball114 and the DC wire 119 a disposed on the installation substrate 122,and the DC bonding wire 118. The GND penetrating electrode 110 connectedto the GND layer 106 is connected to a lower GND pad 116, and connectedto the ground (GND) via the GND solder ball 120 and the installation GNDwire 115 disposed on the installation substrate 122, and the GND bondingwire 121. The shield 113 is connected to the GND penetrating electrode110 b, and connected to the GND. The substrate 112 can be entirelycovered with an insulating layer 111 of thermally-oxidized SiO₂ toprevent the electric short circuit between the wires and electrodes, andthe Si substrate 112. Sizes of the respective portions are indicated inFIGS. 1 to 3B (not to scale).

Materials of the respective portions of the penetrating electrode 102can be as follows. The penetrating electrode 102 a can be formed of Au(3 microns in thickness). The penetrating electrode 102 b can be formedof Au/Cr (3 kÅ/0.5 kÅ). Each of the penetrating electrodes 102 c and 102e can be formed of Cu (300 microns) The penetrating electrode 102 d canbe formed of Cr/Au/Cr (0.5 kÅ/6 kÅ/0.5 kÅ). Further, materials of therespective portions of the GND penetrating electrode 110 can be asfollows. Each of the GND penetrating electrodes 110 a and 110 c can beformed of Cu (300 microns). The penetrating electrode 110 b can beformed of Cr/Au/Cr (0.5 kÅ/6 kÅ/0.5 kÅ). The diameter of each electrodeportion can be set to about 5 microns.

The operation of the oscillator 100 will be described. When the DCcurrent is injected into the RTD 101 through the penetrating electrode102, a THz wave can be inductively emitted by an energy state transitionof electrons due to the resonance tunnel effect in the triple quantumwell structure of the active layer. The THz wave can have a frequency of340 GHz, and a wavelength λ in vacuum of 840 microns. The EMW gain atthis time can be approximately estimated from the measured differentialnegative conductance of −4.1 mS. The thus-emitted THz wave at 340 GHzpropagates as the standing wave with the effective wavelength λ_(g) of570 microns in the resonator formed by the RTD 101, the MSL patchantenna 199, the dielectric portion 107, and the GND layer 106.

The RTD 101 can be activated in a photon assist manner by the THz wavepropagating in the resonator, and the induced emission can be repeated.Therefore, the THz wave at the frequency of 340 GHz is oscillated fromthe antenna 104 at a certain threshold of DC voltage that isapproximately equal to a bias voltage at which the negative differentialresistance can be obtained according to the current-voltagecharacteristic.

In the first embodiment, the penetrating electrode 102 can be disposedat the node of the standing wave of the THz wave in the resonator, whereits electromagnetic field is approximately equal to zero. The diameterof the penetrating electrode 102 can be less than λ_(g)/20. Therefore,the penetrating electrode 102 can supply the DC power to the RTD 101under conditions that the short circuit is established in terms of DCoperation, the circuit is open in terms of high frequency operation, andthe presence of the penetrating electrode 102 is negligible from thestandpoint of the standing wave. Thus, the THz wave at 340 GHz can beoscillated from the oscillator of this embodiment. Further, the lowfrequency noise (less than about several tens MHz) from the DC powersource can be prevented by the grounded shield 113 that extendsapproximately perpendicular to the thickness direction of the substrate112. Accordingly, parasitic oscillation and harmonics oscillation due tothe above factors can be reduced, and the oscillation characteristic canbe improved.

FIGS. 3A and 3B illustrate upper and lower surfaces 300 a and 300 b ofan oscillator array in which the above-discussed oscillators 100 arearranged in an array of 5*3, respectively. For the purpose ofillustration, the installation substrate and so forth are omitted inFIGS. 3A and 3B. The arrangement pitch of the oscillators 100 a to 100 oand the distance between the adjacent oscillators are indicated in FIGS.3A and 3B. For example, the shortest distance between the EMW radiatingportions 194 of one oscillator and its adjacent oscillator is about 416microns, and its layout satisfies the condition that such distanceshould be larger than λ_(g)/2. In FIGS. 3A and 3B, for example, theoscillator 100 h on the upper surface 300 a corresponds to the lower DCelectrode pad 117 h and the lower DC wire 190 h on the lower surface 300b. Further, lower GND pads 116 a, 116 b, 116 c and 116 d are arranged atfour corners of the lower surface 300 b in a horizontally and verticallysymmetrical manner, respectively.

Furthermore, as illustrated in FIG. 2, on the installation substrate122, there can be arranged installation DC wires for other oscillators,for example, the installation DC wires 119 b and 119 c for theoscillators 100 b and 100 c. When the oscillators 100 of this embodimentare densely integrated in an array as described above, the oscillatorarray with enhanced output power can be readily achieved. Further, asmall-sized THz-wave oscillator array with high output power can beprovided. In those structures, high output power is obtained, forexample, by synchronously oscillating the arrayed oscillators in anin-phase manner.

The fabrication technique of the oscillator 100 of this embodiment willbe described. Conventional semiconductor processing techniques can beused.

(1) A first substrate for the EMW radiating portion 194 is prepared. Inthis embodiment, there is prepared an epitaxial substrate in which thequantum well structure of InGaAs and InAlAs for the active layer aregrown on a semi-insulating InP substrate using molecular beam epitaxy(MBE). A layer of Au/Cr (3 kÅ/0.3 kÅ) for a portion of the GND layer 106is formed on the epitaxial surface of the above epitaxial substrate by asputtering technique.

(2) Substrates 112 a and 112 b for the substrate 112 including the DCsupplying portion 198 and the penetrating electrode 102 are prepared.The substrate 112 a is fabricated in the following manner. Thefabrication process of the substrate 112 b is omitted since this is thesame as that of the substrate 112 a.

By a Si Deep RIE technique using conventional photolithography and Boschprocess, penetrating holes with a diameter of 10 microns are formed inthe Si substrate. The holes are formed at locations corresponding to thepositions of the penetrating electrode 102 and the GND penetratingelectrode 110, respectively. Thermally oxidized SiO₂ with a thickness of1 micron is formed on the surface of the Si substrate with thepenetrating holes therein, by a thermal oxidization technique usinghydrogen and oxygen. This thermally oxidized SiO₂ corresponds to theinsulating layer 111.

Thereafter, the penetrating holes in the Si substrate are filled with Cuand TiN by a metal organic-chemical vapor deposition (MOCVD) technique.Further, Cu deposited on upper and lower surfaces of the Si substrate isselectively removed by a chemical mechanical polishing (CMP) technique,and penetrating electrodes are thus formed in the Si substrate. Thosepenetrating electrodes correspond to the penetrating electrodes 102 band 102 c, and the GND penetrating electrode 110 a, respectively.

A layer of Au/Cr (3 kÅ/0.3 kÅ) is formed on upper and lower surfaces ofthe Si substrate with the penetrating electrodes by the sputteringtechnique. The layer of Au/Cr corresponds to the GND layer 106, theshield 113, the lower DC signal line 190, and the lower GND pad 116. Adesired pattern is formed by etching the layer of Au/Cr on upper andlower surfaces of the Si substrate using photolithography and Au/Cr-RIEtechnique. The desired pattern is, for example, an electrode pattern asillustrated on the lower surface 300 b of the oscillator array in FIG.3B. The desired pattern is situated at locations corresponding topositions of the penetrating electrode 102, the GND electrode 110, etc.

(3) The first substrate, and substrates 112 a and 112 b are prepared.The lower surface of the substrate 112 a is caused to face the uppersurface of the substrate 112 b, and their positions are aligned. Thosesurfaces are bonded by Au thermal pressure or solderless bondingtechnique. The substrate is thus fabricated. Then, the epitaxial surfaceof the first substrate is caused to face the upper surface of thesubstrate 112, and their positions are aligned. Those surfaces arebonded by Au thermal pressure or solderless bonding technique. A secondsubstrate composed of the substrate 112 bonded to the first substrate isthus fabricated. Only the InP portion of the second substrate, i.e., thesemi-insulating InP of the first substrate, is selectively removed byetching using polishing technique and wet etching technique. The InGaAssurface is thus exposed.

(4) Using EB (electron beam) drawing technique and RIE technique, theepitaxial layer composed of InGaAs/InAlAs and the like is removed toform a post corresponding to the RTD 101. Then, BCB corresponding to thedielectric portion 107 is deposited by spin-coating. An upper portion ofthe post corresponding to the RTD 101 is exposed by slightly removingthe BCB using the RIE technique. A pattern of Au/Cr is formed byphotolithography and lift-off technique. This pattern corresponds to theMSL patch antenna 199. An opening is formed in the BCB of the dielectriclayer 107 by photolithography and RIE technique to expose the Au layercorresponding to the penetrating electrode 102 b.

A layer of Au (3 microns in thickness) is formed by electroplatingtechnique to form an Au electrode corresponding to the penetratingelectrode 102 a. Using photolithography and lift-off technique, apattern of Au/Cr is formed to form the Au pattern corresponding to theMSL patch antenna 199. The second substrate is placed on theinstallation substrate 122 by flip-chip bonding technique, and thebonded substrates are connected to the external circuit by wire bondingtechnique. The oscillator 100 is thus constructed.

In the above description, Si with characteristics of high aspect andpreferable fine processing capability is used as a material of thesubstrate 112. However, compound semiconductor substrate such as InP andGaAs, quartz, sapphire, ceramics, and the like can also be used as thesubstrate 112. Selection of the substrate material can be an importantfactor for determining the length of the resonator relevant to a desiredoscillation wavelength. This material can be appropriately selected inthe light of the oscillation wavelength, the fine processing capability(precision), and data of the material (dielectric constant and soforth).

Next, details of an oscillator of a second embodiment will be describedreferring to FIG. 4. Specific sizes of various portions of the secondembodiment are indicated in FIG. 4 (not to scale). With respect toportions of upper surface, basic structure and materials of theoscillator similar to those of the first embodiment, illustration anddescription of the oscillator of the second embodiment are omitted.

Similar to the first embodiment, an oscillator 200 of the secondembodiment is designed such that a THz wave at the oscillation frequencyof 340 GHz, namely, λ=840 microns and the effective wavelength λ_(g)=570microns, can be oscillated. The dielectric portion 107 is formed of BCB.Therefore, detailed description of the structure of the EMW radiatingportion 194 is omitted.

In the second embodiment, a penetrating electrode 202 for the DC powersupply, and a GND penetrating electrode 210 for connection to the GNDare in the form of a circular waveguide. A substrate 212 is a Sisubstrate, similar to the first embodiment. In the dielectric portion107 and the substrate 212, there are disposed penetrating electrodeportions 202 a through 202 e for the DC power supply, and GNDpenetrating electrode portions 210 a through 210 c for the connection tothe GND at predetermined locations, respectively. The penetratingelectrode 202 a is connected to the penetrating electrode 202 c via thepenetrating electrode 202 b.

Since the penetrating electrodes 202 c to 202 e are integrally formed inthe fabrication process, they are automatically connected to each other.The penetrating electrode 202 d is connected to the lower DC signal line190 disposed on the lower surface of the substrate 212, through thepenetrating electrode 202 e. The lower DC signal line 190 and the lowerDC electrode pad 117 are connected to the DC wire 119 a disposed on theinstallation substrate 122, via the DC solder ball 114, and connected toexternal apparatuses such as the DC power source (not shown), via the DCbonding wire 118.

The GND layer 106 is connected to the GND penetrating electrode 210 bvia the GND penetrating electrode 210 a. The GND penetrating electrode210 b is connected to the lower GND pad 116 disposed on the lowersurface of the substrate 212, via the GND penetrating electrode 210 c.The lower GND pad 116 is connected to the installation GND wire 115disposed on the installation substrate 122, via the GND solder ball 120,and connected to the ground (GND) via the GND bonding wire 121. Thesubstrate 212 is entirely covered with the insulating layer 111 ofthermally-oxidized SiO₂ to prevent the electric short circuit betweenthe wires and electrodes, and the Si substrate 212. Sizes of therespective portions are indicated in FIG. 4 (not to scale).

Materials of the respective portions of the penetrating electrode 202areas follows. The penetrating electrode 202 a is formed of Au (3microns in thickness). The penetrating electrode 202 b is formed ofAu/Cr (3 kÅ/0.5 kÅ). Each of the penetrating electrodes 202 c, 202 d and202 e is formed of Pd (300 microns in depth, and 1 micron in thickness).Further, all materials of the respective portions 210 a, 210 b and 210 cof the GND penetrating electrode 210 are Pd (300 microns in depth, and 1micron in thickness). Inner diameters of waveguides of the penetratingelectrode 202 and the GND penetrating electrode 210 are set to about 5microns.

The operation of the oscillator 200 of the second embodiment is similarto that of the first embodiment. In the second embodiment, thepenetrating electrode has the waveguide structure, so thatelectromagnetic waves at low frequencies less than the cutoff frequencycan be shielded. Thereby, the low frequency noise (less than aboutseveral tens MHz) from the DC power source can be intercepted.Accordingly, parasitic oscillation and harmonics oscillation due to theabove factors can be prevented, and the oscillation characteristic canbe improved.

The oscillator 200 can be readily fabricated by a method similar to thefabrication method of the first embodiment. In this method, Pd is formedusing an electroless plating method, in place of formation of thepenetrating electrodes by MOCVD.

As described above, in the second embodiment, electromagnetic waves atlow frequencies less than the cutoff frequency can be intercepted by thewaveguide with a cavity in the penetrating electrode, and the lowfrequency noise from the DC power source can be reduced, leading toimprovement of the oscillation characteristic. It is also possible toenhance the shielding effect by loading the above cavity of thewaveguide with resin, or the like.

Next, details of an oscillator of a third embodiment will be describedreferring to FIGS. 5, 6A and 6B. FIG. 5 is a cross-sectional viewillustrating the third embodiment, in which a lens substrate 501 with acurve such as a semi-sphere or an ellipsoid is placed on the oscillator.FIGS. 6A and 6B are plan views illustrating oscillators with lensstructures of two types, respectively. Specific sizes of the lenssubstrate 501 are indicated in FIG. 5 (not to scale). Illustration anddescription of portions of the third embodiment similar to those of thefirst embodiment are omitted.

As illustrated in FIG. 5, in the oscillator 500 of the third embodiment,the semi-spherical lens substrate 501 is placed on the oscillator 100designed such that the THz wave at the frequency of 340 GHz can beoscillated as described in the first embodiment. The radius of thesemi-spherical lens portion is 10 mm, and the thickness of its substrateis 100 microns. FIGS. 6A and 6B illustrate two arrangements of the lenssubstrate. In the structure of FIG. 6A, an array of micro-lenses 601 aredisposed at the same pitch as that of an array of oscillators 300. Inthe structure of FIG. 6B, a single silicon semi-spherical lens 602 isdisposed on the array of oscillators 300.

In the third embodiment, the lens substrate 501 is formed of resin ofcycloolefin group that has a relatively small dielectric loss for a THzwave, and an excellent processing capability. The lens substrate 501 isproduced by molding technique using a conventional metal mold for fineprocess machining. A material with a small loss for the THz wave ispreferable as the lens material that is selected in the light of thelens profile and the processing capability. Inorganic materials, such assilicon, ceramics, and glass, organic materials, such as polyethyleneand Teflon (trade mark), and the like can also be used.

The lens substrate 501 is connected to the oscillator 100 through aspacer 502. In this embodiment, the spacer 502 is a double-stickadhesive tape made of polyethylene telephthalate (PET) that has a highprocessing capability, and is 100 microns in thickness. After thesubstrate of the oscillator 100 and the lens substrate 501 are alignedto the spacer 502, they are subjected to thermal solderless bonding at80 degrees Celsius. The spacer 502 serves as a spacer layer fordetermining the distance between the upper surface of the substrate ofthe oscillator 100 and the lens substrate 501, as well as a physicalconnecting layer. By regulating the plate thickness of the spacer 502,the coupling efficiency of an electromagnetic wave generated from theoscillator 100 to the lens substrate 501 can be optimized.

As another material of the spacer 502, it is possible to use siliconthat has an excellent capability of controlling the above platethickness. In this case, an adhesive resin is used, and the device isfabricated by a die bonding technique. Further, when the spacer 502 ismade of an epoxy adhesive or the like, the lens substrate 501 can bedirectly bonded.

In the EMW oscillator with the arrangement of the penetrating electrodefor the DC power supply, there is no need of arranging members for thepower supply on the upper surface of the oscillator. Therefore, asdescribed above, the lens substrate 501 can be placed on the uppersurface of the oscillator 100. Accordingly, an electromagnetic waveradiated from the oscillator 100 can be coupled to the curved lens, suchas the semi-spherical lens and the ellipsoidal lens, be shaped, and beefficiently picked out.

Except as otherwise disclosed herein, the various components shown inoutline or in block form in the figures are individually well-known andtheir internal construction and operation are not critical either to themaking or using of the present invention or to a description of the bestmode of the invention.

While the present invention has been described with respect to what ispresently considered to be the embodiments and examples, it is to beunderstood that the invention is not limited to the disclosedembodiments and examples. The present invention is intended to covervarious modifications and equivalent arrangements included within thespirit and the scope of the appended claims.

This application claims priority from Japanese Patent Application No.2006-150924, filed May 31, 2006, and Japanese Patent Application No.2006-349233, filed Dec. 26, 2006, the contents of which are herebyincorporated by reference.

1. An oscillator for emitting terahertz waves, the oscillatorcomprising: a substrate, the substrate having a first surface, and asecond surface extending on a side opposite to a side on which the firstsurface extends; a layer including a gain portion; a first electrode anda second electrode which are electrically connected to the gain portion;an oscillation unit disposed on the first surface of the substrate andformed by sandwiching the layer between the first electrode and thesecond electrode; a third electrode disposed on the second surface ofthe substrate; and a first penetrating electrode penetrating the insideof the substrate and connecting the first electrode and the thirdelectrode such that a direct current is supplied, wherein theoscillation unit radiates terahertz waves generated by supplyingelectric power from the third electrode to the gain portion through thefirst penetrating electrode and the first electrode.
 2. The oscillatoraccording to claim 1, wherein the first penetrating electrode isdisposed at a node of a standing wave of an electromagnetic fieldresonating in the electromagnetic-wave oscillating unit.
 3. Theoscillator according to claim 1, wherein the gain portion comprises anegative resistance element.
 4. The oscillator according to claim 3,wherein the negative resistance element comprises a resonance tunneldiode.
 5. The oscillator according to claim 1, wherein a plurality ofthe oscillators are arranged in an array.
 6. The oscillator according toclaim 1, wherein the oscillation unit is disposed on the first surfaceby bringing the first electrode or the second electrode into contactwith the first surface.
 7. The oscillator according to claim 1, furthercomprising: a fourth electrode disposed on the second surface; and asecond penetrating electrode penetrating the inside of the substrate andconnecting the second electrode and the fourth electrode such that adirect current is supplied.
 8. The oscillator according to claim 7,wherein a reference electric potential for the gain portion is regulatedby grounding the fourth electrode.
 9. The oscillator according to claim1, wherein the oscillation unit includes a radiating portionelectrically connected to the gain portion and is formed such that theterahertz waves are radiated from the radiating portion.